A gas sensor using solid proton conductive membrane is known (Documents 1 and 7). In this gas sensor, a proton conductive membrane is interposed between a pair of electrodes, and water vapor is fed from a water reservoir. After that, the present inventors considered the possibility of applying the structure of the gas sensor using a solid proton conductive membrane to a gas sensor using a liquid electrolyte.
In a gas sensor using a liquid electrolyte, the electrolyte is held in a separator, and the electrolyte is replenished from a reservoir of the electrolyte by means of a wick. As sulfuric acid is used as the electrolyte, a metal housing can not be used. Furthermore, in an atmosphere of high humidity, sulfuric acid will absorb moisture and may overflow from the reservoir.
Document 2 proposes a liquid electrochemical gas sensor using no wick. Here, sulfuric acid is stored in a water reservoir, and sulfuric acid is arranged to absorb moisture when the humidity is high and release moisture when the humidity is low, thus substantially maintaining the humidity at a constant level in the gas sensor. As a result, the electrolyte in the separator is prevented from drying. Document 3 proposes to set a deliquescent salt such as LiCl in a water reservoir so as to substantially keep the humidity at a constant level in the gas sensor. When sulfuric acid or a deliquescent salt is used, however, there is a possibility that in an atmosphere of high temperature and high humidity or the like the liquid electrolyte may overflow from the water reservoir.
Document 4 discloses a separator wherein colloidal silica and PTFE (polytetrafuluoloethylene) are carried on a paper-like glass filter. Here, it is understood that hydrophilic channels for maintaining the liquid electrolyte are provided by colloidal silica, and hydrophobic channels for diffusing gas are provided by PTFE.
Document 5 discloses an O2 sensor using a liquid electrolyte of KOH or H2SO4, and it is reported that the characteristics will drift when KOH is used.    Document 6 discloses a CO sensor using an MgSO4 aqueous solution.    Document 1: WO 02/097420A1    Document 2: WO 01/14864A1    Document 3: U.S. Pat. No. 5,958,200    Document 4: U.S. Pat. No. 4,587,003    Document 5: U.S. Pat. No. 5,240,893    Document 6: U.S. Pat. No. 5,302,274    Document 7: U.S. Pat. No. 6,200,443